Turk J Phys(2020) 44: 77 – 84© TÜBİTAKdoi:10.3906/fiz-1910-15

Turkish Journal of Physics

http :// journa l s . tub i tak .gov . t r/phys i c s/

Research Article

Anisotropic magnetoresistance and planar Hall effect in magnetoresistiveNiFe/Pt thin film

Erdem DEMİRCİ∗

Department of Physics, Faculty of Science, Gebze Technical University, Kocaeli, Turkey

Received: 16.10.2019 • Accepted/Published Online: 28.01.2020 • Final Version: 12.02.2020

Abstract: The magnetoresistive NiFe/Pt sensor was fabricated by using standard microlithography and sputter depo-sition techniques and then the magnetic and magnetotransport properties were investigated. The magnetic propertiesof the sample show that the easy axis of magnetization prefers a predefined direction along the Hall bar. Moreover,the sample has uniaxial magnetic anisotropy due to the growth-induced magnetic anisotropy. On the other hand, theAnisotropic magnetoresistance (AMR) and the Planar Hall effect (PHE) of the sample were investigated in detail be-tween 0°and 360°. The angle-dependent AMR and PHE signals at the in-plane magnetic field of constant magnitudeintersect at certain angles. At these angles, the presence of unusual peaks and valleys was observed. This is proof thatAMR and PHE curves had strongly affected each other at certain angles.

Key words: Magnetoresistive sensor, planar Hall effect, anisotropic magnetoresistance, NiFe

1. IntroductionIn recent years, the magnetoresistive sensors based on anisotropic magnetoresistive (AMR) have attracted greatattention owing to their high sensitivity at weak magnetic fields, the flexibility of the design, and compatibilitywith standard microelectronics technology [1–5]. The magnetoresistive sensors are commonly used for magneticbead detection [6], biosensors and biochips [7, 8], applications to microcompass [9], and flexible magnetic sensors[10, 11]. The physical phenomenon of the magnetoresistance effect is related to the change in the resistance ofthe magnetic conductor under external magnetic field [12]. The change in the resistance (R) is experimentallyexpressed as

R=R0+∆R cos2 θ (1)

Here, R0 is the electrical resistance in the absence of a magnetic field, ∆R is the difference in the resistance ofthe magnetic conductor and θ is the angle between the current (I) and magnetization (M) .

It is known that the AMR effect arises from the anisotropic scattering of conductive electrons in aferromagnetic material [13]. In this phenomenon, the resistance is at the maximum when M and I are parallel(R//) or it is at minimum when M and I are perpendicular with each other (R⊥) . The AMR effect within-plane magnetization is described by Eq. (2) [13]. However, unlike the AMR in longitudinal geometry, themagnetoresistance measurement in transverse geometry causes the electric field perpendicular to the current.The phenomenon in transverse geometry is called the planar Hall effect (PHE) and it is expressed by Eq. (3)[13]. Although the PHE output voltage is very low compared to the AMR output voltage, the planar Hall effect∗Correspondence: e.demirci@gtu.edu.tr

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is considerably reduced the temperature drift and increased the resolution of magnetoresistive sensors [14]. Fora single domain ferromagnetic conductor, electric fields (E) of AMR and PHE are expressed as follows:

EAMR=jρ⊥+j(ρ||−ρ⊥) cos2 θ (2)

EPHE= j(ρ||−ρ⊥) sin θ cos θ (3)

Here, ρ// and ρ⊥ are the resistivities for the current parallel and perpendicular to the magnetization orien-tations, respectively. According to Eq. 3, PHE voltage is proportional to the sin2θ angle between M andI . Therefore, PHE voltage is extremely sensitive rather than AMR voltage to detect small deviations in themagnetization from the current direction [15, 16]. Also, the background signals existing in the AMR are notobserved in the PHE. By considering these disadvantages of AMR compared to PHE, the magnetoresistivesensor based on AMR requires high sensibility to the deviation of magnetoresistance for novel technology.

In the present work, the magnetoresistive Py (10 nm)/Pt (5 nm) sensor was fabricated with Hall bar andcontinuous film structures on SiO2 /Si. Then, AMR and PHE measurements of the sensor were performed in anapplied external magnetic field at a different angle to compare the effect of their signal responses on each other.To observe a small deviation of magnetoresistance based on current was examined in detail by using PHE andAMR techniques.

2. Experiment and discussion2.1. FabricationThe magnetoresistive sensor based on AMR was designed and fabricated using standard microlithography andsputter deposition techniques. First, Hall bar and continuous patterns were realized on SiO2 /Si substrates byoptical lithography. Before the lithography processes, the SiO2/Si substrate was cleaned by sequentially rinsingin acetone, isopropanol, and deionized water. After that, a positive photoresist (AZ 1505) was coated onto thecleaned SiO2 /Si substrate by using a spin coater. To form a uniform photoresist with a thickness of 500 µm,coating processes were carried out at 4000 rpm for 50 s. The photoresist was baked on the hot plate at 110 °Cfor 60 s. Then, it was exposed at constant intensity ultraviolet light (365 nm, i -line) for 0.5 s and developedby using AZ 726 MIF for 60 s. After the patterning process, Py (10 nm)/ Pt (5 nm) layers were deposited onSiO2 /Si by using magnetron sputtering. Py and Pt layers were deposited at room temperature by using 20 W(RF) and 10 W (DC) powers, respectively. The base pressure level of the sputter chamber was 6 ×10−9 mbarand the working pressure was kept at 5 ×10−3 mbar during the deposition. After that, the sample was soakedin acetone for 10 min and residual photoresist on SiO2/Si substrate was lifted off. The microscope image of thesample consists of Hall bar and continuous parts are as shown in Figure 1.

Figure 2 shows the measurement geometry for the magnetotransport experiments. In this geometry, theeasy axis of the sample is parallel to I . Here, θ is the angle between I and the M , and θH is the anglebetween I and the external magnetic field (Hext) . Magnetoresistive experiments were performed by usinga magnetotransport system. A typical current strength was supplied by a Keithley 2400 source meter andthe current was fixed at 50 µA for all magnetoresistive experiments. The magnetic field-dependent voltagewas measured by a Keithley 2182 nano voltmeter. The angle-dependent magnetoresistive experiments werecarried out between 0°and 360°by using an integrated stepper motor. During the magnetoresistive experiments,a constant current was passed through the long direction of the Hal bar between 1 and 6 terminals for all

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magnetotransport measurements. Then, the AMR voltage between 3 and 4 terminals (V3− 4) and PHE voltagebetween 2 and 3 terminals (V2− 3) were recorded by using nanovolt meter while the in-plane magnetic fieldwas swept. The distance is 200 µm between the magnetoresistance terminals (V3− 4) , and 25 µm between theplanar Hall terminals (V2− 3) . The long axis of the sample in the Hall bar is prepared with 8:1 aspect ratio toinduce shape anisotropy.

Figure 1. Photograph of the sample mounted on a printedcircuit board (PCB). The sample consists of Hall bar (top)and continuous film (bottom). The contact pads and PCBterminals were connected with gold wires by using wirebonder. The inset is a microscope top-view image of themagnetoresistive Py/Pt sensor.

Figure 2. Schematic representation of the Hall bar geom-etry with the 6 terminals. The width of the Hall bar is 25µm and the length is 200 µm. A constant current of 50 µAwas passed through 1–6 terminals by using Keithley 2400source meter. The voltage is measured 2–3 terminals forPHE and 4–5 terminals for AMR by using Keithley 2182nanovoltmeter.

3. Results and discussionMagnetic hysteresis loops of the sample were measured by using a magneto-optical Kerr effect (MOKE) at roomtemperature. First, angle-dependent longitudinal MOKE (L-MOKE) measurements were carried out from 0°to360°by using a stepper motor. L-MOKE hysteresis loops of the Si/ SiO2 / Py/ Pt were used for the determinationof the angular dependence of the normalized remanent magnetization (MR /MS) as shown in Figure 3. Thesample exhibits in-plane uniaxial magnetic anisotropy. The growth-induced anisotropy is the major cause ofthis behavior of the sample since direction of magnetization is closely related to the growth conditions in themagnetron sputtering systems. During deposition, the substrate is deposited with atoms from an oblique angle;thus, magnetization can be oriented in a specific direction. Here, the easy axis of the magnetization prefers toextend along the Hall bar of the sample due to the growth-induced magnetic anisotropy [17,18]. In this way,M and Iwill be parallel to each other. Figure 3 insets show hysteresis loops of the sample taken at the anglesof 0°and 90°. Easy and hard axes of the sample are at φ = 0°(blue) and φ = 90°(red), respectively.

In magnetoresistive experiments, two types of measurements were performed to compare the relationshipbetween the resistance and applied magnetic field. The first is the dependence of AMR and PHE voltages onthe magnetic field while the sample is at a fixed angle. The second is the dependence of the AMR and PHEvoltages on the angle at the fixed magnetic field.

Figure 4 shows the dependence of AMR on the magnetic field at specified angles. First, while the appliedexternal magnetic field was parallel to the current, the voltage value was simultaneously recorded. Resistancemeasurement of the sample as a function of the field reveals typical AMR behavior as shown in Figure 4a. Then,

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Figure 3. Angle-dependent behavior of the normalized remanent magnetization (MR /MS) of the magnetoresistivePy/Pt sensor. Easy and hard axes of the sample are at φ = 0°(blue) and φ = 90°(red), respectively.

the sample was rotated by 45° at in-plane geometry. Thus, the φH angle between the external magnetic fieldand the current was 45°. However, in this angle, the sample exhibited unusual AMR behavior as in Figure 4b.At the high magnetic fields which are required to saturate the sample, the Py magnetic moments are alignedalong the external magnetic field direction as shown in Region 1 on the inset in Figure 4b. While the Hext

was reduced beyond Region 2, the resistance was increased. However, when the magnetic field was furtherreduced, the resistance of the sample was abruptly decreased due to a greater scattering for a transport electronas shown in Region 3 on the inset in Figure 4b. When the external magnetic field was zero, the RAMR wasmaximum. At this point, the magnetic moments of the sample begin to rotate. Then, while the magnetic fieldwas increased in a negative direction, the resistance of the sample was again decreased throughout region 4on the inset in Figure 4b. Further increase in the applied external magnetic field caused a typical decrease inRAMR as shown in Region 5. In the negative high magnetic fields, the magnetic moments of the sample wererotated in the opposite direction to the external magnetic field direction as shown in Region 6 on the inset inFigure 4b. However, in case the applied external magnetic field was perpendicular to the current, unusual peaksin Regions 3 and 4 on the inset in Figure 4b almost vanished as shown in Figure 4c.

To further investigate the unusual peaks in the AMR measurements, PHE measurements were performedat various angles. Then AMR and PHE measurements were compared in terms of magnitude. During thePHE measurements, a constant current of 50 µA was passed through between 2 and 7 terminals in the longdirection of the Hall bar. Then, PHE voltage was recorded by using nanovoltmeter between 1 and 3 terminals(V1− 3) while the in-plane magnetic field was swept. Figure 5 shows the dependence of PHE voltage on themagnetic field at specified angles. First, the applied external magnetic field was kept parallel to the currentand perpendicular to voltage as shown in Figure 5a. The resistance of the sample as a function of the magneticfield exhibited typical PHE behavior. Then, the sample was rotated by 45°at in-plane geometry. The φH anglebetween the external magnetic field and the current was 45°while RPHE measurement of the sample was carriedout as shown in Figure 5b. Figures 5a and 5b were compared in terms of resistance and it was observed thatthe resistance increased significantly while the sample was rotated by 45°. Later, the applied external magneticfield was kept perpendicular to the current parallel to the voltage as shown in Figure 5c. Unlike previous PHEsignals in Figures 5a and 5b, the nontypical PHE signal was observed in Figure. 5c.

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Figure 4. AMR characterizations of the sample. a) applied external magnetic field is parallel to the current, b) the φH

angle between the external magnetic field and current is 45°, and c) applied external magnetic field is perpendicular tothe current.

Figures 4b and 5b were compared to investigate the unusual peaks in the AMR signal and the unusualvalleys of the PHE signal as shown in Figure 6. It was seen that unusual peaks in the AMR signal and valleysof the PHE signal were overlapped. In addition to this, the amplitude of unusual peaks in the AMR signal wasaffected due to the PHE signal at different angles.

The angle-dependent AMR and PHE measurements at the in-plane magnetic field of constant magnitudeof 300 Oe were performed to find the maximum amplitude of unusual peaks in the AMR signal. Figure 7 showsthe angle-dependent AMR and PHE measurements. The AMR and PHE resistance values were normalized tothe resistance of the sample measured at magnetic saturation. Both AMR and PHE curves are matched verywell with theoretical predictions. More importantly, there are a few intersection points at certain angles. Theintersections of RAMR and RPHE are shown in pink circles as shown in Figure 7. At these intersection points,AMR and PHE curves had strongly affected each other.

4. ConclusionThe NiFe/ Pt magnetoresistive sensor was prepared to compare PHE and AMR effects by using standardmicrolithography and sputter deposition techniques. Then, the L-MOKE measurements were performed forthe sample at room temperature. The L-MOKE measurements show that the sample has an in-plane uniaxial

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Figure 5. PHE characterizations of the sample. a) applied external magnetic field is parallel to the current, b) the φH

angle between the external magnetic field and current is 45°, and c) applied external magnetic field is perpendicular tothe current.

Figure 6. Direct comparison of the AMR (blue) and PHE (red) curves of the magnetoresistive Py/ Pt sensor at φ =45°.

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Figure 7. Normalized RAMR (blue) and RPHE (red) curves are plotted as a function of the orientation of the appliedfield. The RAMR and RPHE curves intersect at specified angles shown in pink circles.

magnetic anisotropy. Also, the easy axis of magnetization extends along the Hall bar of the sample due to thegrowth-induced magnetic anisotropy. Magneto-transport measurements were carried out along with magneticmeasurements. It was observed that AMR and PHE curves have unusual peaks and valleys at the same angle,respectively. In order to determine these angles, the angle-dependent magneto-transport measurements weretaken. The angle-dependent AMR and PHE measurements at the in-plane magnetic field of constant magnitudeare compatible with theoretical predictions. The measurements also showed that AMR and PHE curves intersectat certain angles. At these angles, the magnetic-field-dependent measurements of AMR and PHE at specifiedangles showed unusual peaks and valleys. It is proof that AMR and PHE curves had strongly affected eachother at certain angles.

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